fix: correct the logic to fetch transactions

[moreal]

This pull request fixes the logic to fetch transactions. To do it, I made some changes:

• Validate GetTransactionsResponse manually. It validates responses like below:

  {
  	"data": {
  		"transaction": null
  	},
  	...
  }

  {
  	"data": {
  		"transaction": {
  			"ncTransactions": null
  		}
  	},
  	...
  }

  {
  	"data": {
  		"transaction": {
  			"ncTransactions": [
  				{
  					...,
  					"actions": [
  						null
  					],
  					...
  				}
  			]
  		}
  	},
  	...
  }

• Checks "errors" field is not null because there will be errors field when there were any errors while resolving the query, according to the GraphQL spec, https://spec.graphql.org/October2021/#sec-Errors.
• Expire cached value after 30 seconds from writing. Though it tried to validate the responses, it can cache invalid response unfortunately. So it expire the cached value after 30 seconds from writing.

[moreal]

In internal chat, I had a conversation with @boscohyun about how to handle errors field in GraphQL response because current HeadlessGQLClient's behavior doesn't follow GraphQL's spec completely.

However, we agreed that HeadlessGQLClient's current behavior is enough in current situation, so I'll merge this pull request and continue the discussion in #468.